Question Details

Consider a 3 ×3matrix A whose (i,j)-th element, ai,j = (i-j). Then the matrix A will be

Options

A

null.

B

symmetric.

C

skew-symmetric.

D

unitary.

Show Answer

Correct Answer :

Option C

skew-symmetric.

Solution :

The correct answer is skew-symmetric.

To determine the nature of the matrix A, we need to examine the relation between its elements ai,j and their transposed counterparts aj,i.

The elements of the 3×3 matrix A are given by the formula:
ai,j=(i-j)3
for all indices i and j.

Now, let us find the expression for the transposed element aj,i by swapping the row index i and the column index j:
aj,i=(j-i)3
We can factor out -1 from the term inside the parenthesis:
j-i=-(i-j)
Substituting this back into the expression for aj,i, we get:
aj,i=[-(i-j)]3
Since the exponent 3 is an odd integer, we have (-1)3=-1. Therefore:
aj,i=-(i-j)3
Comparing this with our original formula for ai,j, we have:
aj,i=-ai,j
for all i and j.

By definition, a square matrix A is skew-symmetric if its transpose satisfies AT=-A, which is equivalent to the element-wise condition:
aj,i=-ai,j
Since this condition holds true for all elements of the matrix, the matrix A is a skew-symmetric matrix.

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